Sulphur isotope composition of dissolved sulphate in the Cambrian–Vendian aquifer system in the northern part of the Baltic Artesian Basin

•δ18OSO4 suggests that the sulphate was derived from oxidation of sulphide minerals.•The enrichment of δ34SSO4 is accompanied by depleted δ13C values of the DIC.•The bacterial activity may have caused the enrichment of δ34SSO4.

The groundwater in the Cambrian–Vendian aquifer system with its δ18Owater values of about − 22‰ (VSMOW) and a low radiocarbon concentration is of glacial origin from the Last Ice Age. Earlier surveys have highlighted a negative co-variance of sulphate and bicarbonate content in the groundwater of the Cambrian–Vendian aquifer system, whereas the most depleted dissolved inorganic carbon δ13C values have been measured mainly in groundwater samples with the lowest sulphate concentrations. In this paper we studied the origin of sulphate and the factors controlling the sulphur and carbon isotope geochemistry in the aquifer system. Direct sources of sulphate were not found, but relying upon δ18OSO4 measurements we suggest that the sulphate originates from oxidation of sulphide minerals whereas the δ34S of the dissolved SO42 − in the groundwater is more enriched than the δ34S of the surrounding rocks. We show that bacterial activity may have caused the enrichment of δ34S of sulphate.